Spectroscopic and magnetochemical studies of several electron transfer proteins are proposed. The principal objectives are to (1) examine the catalytic significance of steady-state oxygen intermediates in the Rhus laccase reaction, (2) spectroelectrochemically characterize Rhus laccase, (3) study intersite interactions through examination of the magnetic behavior of the metal sites in cytochrome aa3, cytochrome cd, and human ceruloplasmin, and (4) spectroscopically characterize cytochrome cd and nickel(II)-substituted blue copper proteins.